The invention relates to a device for the protection of an electrode during the resistance welding of workpieces, particularly metal sheets, including a strip placed over the electrode, preferably in a manner displaceable relative to the same, as well as a spot-welding tool for the resistance-welding of workpieces, particularly metal sheets, including at least one electrode as well as an optional winding mechanism for winding and unwinding a strip for the protection of said at least one electrode. Furthermore, the invention relates to a resistance-welding method by which two workpieces, particularly metal sheets, are welded with each other by the aid of spot-welding tools, wherein at least two electrodes are pressed against each other and powered with energy with the workpieces, particularly metal sheets, being interposed.
At present, metal sheets or other workpieces made of aluminum alloys and magnesium as well as galvanized or coated steel sheets—partially in high-strength quality—are used to an increasing extent, for instance, in body-making. Apart from advantages like weight saving and good corrosion resistance, these materials or their coatings cause problems during joining by spot-welding. Above all, the tool life quantity of the spot-welding electrodes employed, which is strongly reduced as compared to that of blank steel sheets, has adverse effects. The high wear of the electrodes involves high costs due to the frequent reworking of the electrode contact surfaces and the frequent electrode exchanges resulting therefrom as well as a reduced welding quality, particularly prior to an electrode reworking process or prior to an electrode exchange.
For the protection of spot-welding electrodes from contamination or an electrode pick-up by the workpiece to be welded, it is known to insert a metal film in strip-form between the electrode and the workpiece. In order to ensure safe strip feeding, it is in fact necessary to prevent any adherence of the strip to the electrode contact surface.
In this respect, a device for the protection of electrodes during spot-welding is known from EP 0 830 915 B1, in which a strip is drawn by the aid of an unwinding mechanism over the electrode to be protected. The strip consists essentially of a copper-nickel alloy, or pure nickel, and has a thickness of 0.02 to 0.05 mm. In order to ensure an extended tool life quantity, the electrode and, in particular, the electrode cap is coated with silver, or a silver metal-oxide, or provided with an appropriate insert.
Furthermore, a strip for the protection of an electrode in a resistance-welding method is known from U.S. Pat. No. 5,552,573 A, in which the strip is comprised of a base material to which one layer is each applied on either side, wherein the layers may be made of the same or different materials. The base material has a thickness of 0.02 to 1 mm and is comprised of iron, steel, copper or a copper alloy. The layers applied have thicknesses ranging from 1 μm to 100 μm and may be comprised of nickel, titanium, niobium, molybdenum, tungsten, chromium, cobalt or alloys thereof. Such strips have the drawback of involving extremely high production expenditures due to coating or alloying, which lead to extremely high costs in the production of such strips. Another very important disadvantage resides in the fact that a plurality of combinations of the most different materials may be envisaged such that a demanding and expensive storage of such combination strips for the most different applications is necessary, because the user has no way to compose the different combinations on his own.
An application of strips for the protection of electrodes in spot-welding methods are further known from DE 197 54 546 C1, JP 10 029 071 A, JP 08 118 037 A, JP 04 322 886 A or JP 05 192 774 A. There, the strip, which is supplied and discharged via a winding mechanism, is positioned in the spot-welding tools above the electrode as a protection means against contacting of the electrode and the workpiece or structural component, so that the strip will come to lie on the workpiece or structural component in a spot-welding process.
In the methods according to DE 197 54 546 C1 and JP 04 322 886 A, it is disadvantageous that the strip is drawn over the electrode during conveyance, thus causing a high electrode wear on account of the friction occurring between the electrode and the strip.
The methods according to JP 10 029 071 A and JP 08 118 037 A involve the disadvantage that a highly complex structure is required for the conveyance of the strip and, in particular, its supply and discharge, so that spot-welding tools of this kind are hardly usable in practice. It is only feasible by such apparatus to weld simple metal sheets. An application in spot-welding robots, particularly for the automotive industry, is, however, impossible, because this would require small, compact spot-welding tools.